Learn Italian: Non aprire quella porta!

In Italian, when you give a negative command to one person informally (the tu form), you use non plus the infinitive of the verb. So instead of non apri, you must say non aprire.

If I wanted to tell a group of friends not to open the door, would I still use aprire?

No! The rule of using the infinitive for negative commands only applies to the singular tu. For a group (the voi form), you just put non in front of the normal conjugation: Non aprite quella porta!

Why is the word for "that" quella here instead of quel?

The word for "that" changes depending on the gender of the noun it describes. Because porta (door) is feminine singular, you use quella. If you were talking about a masculine object, like a book (libro), you would use quel.

Could I say questa porta instead? What is the difference?

Yes, but it changes the meaning. Questa porta means "this door" (close to the speaker), whereas quella porta means "that door" (further away).